As a transmission system of a transmitting apparatus, there is a digital coherent optical transmission system, for example. In the digital coherent optical transmission system, a multi-level modulation system of an optical signal is set in a transmitting apparatus according to the transmission speed (line rate) of the optical signal if the baud rate is constant.
For example, assuming that the baud rate is approximately 32 (Gbaud), if the line rate is 100 (Gbps), dual polarization (DP)-quadrature phase shift keying (QPSK) is selected as the modulation system. If the line rate is 200 (Gbps), DP-16-quadrature amplitude modulation (QAM) is selected as the modulation system.
In recent years, due to improvement in the performance of digital signal processors (DSPs), analog-to-digital converters, and digital-to-analog converters, transmitting apparatuses that allow change in the baud rate in a range of 32 to 64 (Gbaud), for example, have been researched and developed. For this kind of transmitting apparatus, it is desired to select and set the optimum combination of the baud rate and the multi-level modulation system in terms of optical transmission characteristics. As a related art, a point that compensation operation of optical transmission characteristics is controlled by using the Q-factor is disclosed in Japanese Laid-open Patent Publication No. 2005-64905, for example.
When a multi-level degree of the multi-level modulation system is higher, the interval between symbols in the constellation of an optical signal becomes shorter. Thus, room for the phase and amplitude of the optical signal on the receiving side decreases, and optical signal-to-noise ratio (OSNR) tolerance deteriorates due to the influence of noise from an optical amplifier and so forth. To address this, the multi-level modulation system with a low multi-level degree may be selected so that the OSNR tolerance may become sufficient. However, as the multi-level degree becomes lower, the number of bits per symbol becomes smaller, and therefore it is desired to set a higher baud rate when implementation of a given line rate is intended.
However, as the baud rate becomes higher, the bandwidth (spectrum) of the optical signal becomes wider. Therefore, for example, the influence of pass band narrowing (PBN) due to a wavelength filter of a wavelength selective switch (WSS) on the transmission route of the optical signal increases. As above, a trade-off exists between the baud rate and the multi-level modulation system. Therefore, there is a problem that it is difficult for a person in charge of network design to manually select the optimum combination of the baud rate and the multi-level modulation system.
In view of the above, it is preferable to provide a network design apparatus and a network design method that allow proper network design in consideration of pass band narrowing and OSNR.